EP3394535B1 - Method and device for the cryogenic decomposition of syngas - Google Patents

Description

The invention relates to a process for the cryogenic decomposition of a feed gas consisting predominantly of hydrogen and carbon monoxide and containing methane, which is partially condensed by cooling in order to obtain a first liquid phase consisting largely of carbon monoxide and methane and containing hydrogen, from which a second liquid phase is produced by separating hydrogen in an H ₂ separation column heated by a circulating heater, from which second liquid phase a carbon monoxide-rich gas phase is obtained in a CO/CH ₄ separation column with a purity that allows its release as a carbon monoxide product.

Furthermore, the invention relates to a device for carrying out the method according to the invention.

Such a method and such a device are known from the publication FR-A-3 011 069 and the technical article "Cryogenic Gas Separation" by Linde AG.

Processes of this type have been known to those skilled in the art for many years as so-called condensation processes. They are preferably used to decompose synthesis gases obtained by partial oxidation and therefore have a high carbon monoxide and low methane content. Provided the feed gas is sufficiently cooled, the condensation process allows the production of a carbon monoxide product with a yield of more than 90%, which has a methane content of less than 400 ppm and can therefore be used, for example, to produce monoethylene glycol without further purification.

In order to provide the peak cooling required for the process and to generate reflux at the top of the CO/ _CH4 column, a state-of-the-art cooling circuit is used, which uses either externally supplied nitrogen or internally generated carbon monoxide as the refrigerant. Both options are complex and represent a significant cost factor, significantly impacting the economic viability of gas separation.

For the carbon monoxide cycle, a portion of the CO/CH ₄ separation column and the process streams to be cooled are heated The carbon monoxide-rich gas phase is compressed, liquefied to counteract process streams that need to be heated, and expanded to the top of the CO/ _CH4 column for cooling. A portion of the resulting liquid phase forms column reflux, which achieves the required purity of the carbon monoxide product, while the remainder is further expanded to provide peak cooling for the process.

A nitrogen cycle is also used according to the state of the art to provide the peak cooling for the process and to generate a reflux for the CO/CH ₄ separation column, which is equipped for this purpose with a condenser which, cooled with liquid nitrogen, provides a temperature difference at the top of the column to drive an internal carbon monoxide reflux.

Both refrigeration circuits are driven by multi-stage compressors. While a two-stage, comparatively inexpensive compressor can be used in a nitrogen circuit, a carbon monoxide compressor is considerably more expensive. This is due, firstly, to the fact that a carbon monoxide compressor must be designed with at least three compressor stages to prevent thermal decomposition of carbon monoxide and the resulting soot deposits. Secondly, it must be explosion-proof and operated in a specially secured area to prevent escaping carbon monoxide from causing harm to people and equipment. The costs for a compressor in a carbon monoxide circuit are therefore up to 50% higher than for a compressor suitable for driving a corresponding nitrogen circuit.

However, the cost advantages of the nitrogen cycle resulting from the compressor are partially offset by the necessary condenser at the top of the CO/CH ₄ separation column and the resulting higher energy requirement compared to a carbon monoxide cycle.

The object of the present invention is therefore to provide a method of the generic type and a device for carrying it out, which allow a carbon monoxide product to be obtained at reduced costs compared to the prior art.

This task is solved by withdrawing a low-methane stream from the _H2 separation column and then feeding it as reflux at the top of the second separation column.

During the separation of hydrogen from the first liquid phase, the second liquid phase, which consists largely of carbon monoxide and methane, collects in the bottom of the _H2 separation column, while a hydrogen-rich gas phase is withdrawn overhead. A gas phase containing hydrogen, carbon monoxide, and methane, generated by the circulation evaporator, rises from the bottom space and is brought into intensive contact with the first liquid phase, which is conducted in countercurrent, via separation stages. Because predominantly methane and carbon monoxide are backwashed from the gas phase and hydrogen is stripped from the liquid phase, the compositions of the material streams change continuously in the direction of flow. While the proportions of carbon monoxide and - even more so - methane decrease in the gas phase and the proportion of hydrogen increases, the corresponding proportions in the liquid phase develop in the opposite direction. Material streams with different compositions can therefore be withdrawn from the first _H2 separation column at different heights.

The invention takes advantage of the fact that at least one stream is present within the _H2 separation column with a composition suitable for use as reflux in the CO/ _CH4 separation column. According to the invention, this stream is low in methane and, in particular, has a low hydrogen content.

Preferably, the low-methane stream is withdrawn in gaseous form from the H ₂ separation column and subsequently cooled and liquefied by cooling against process streams to be heated and/or a refrigerant before being introduced as reflux into the CO/CH ₄ separation column.

Experience has shown that the gas phase in the _H2 separation column upstream of the sixth practical separation stage has a suitable composition, so that the low-methane stream is withdrawn from the _H2 separation column in gaseous form before the sixth separation stage. Downstream of the sixth practical separation stage, the methane content of the gas phase continues to decrease, but the hydrogen content is too high for use in the CO/ _CH4 separation column.

Preferably, the discharge point for the low-methane gas phase is located between the bottom space and the third practical separation stage of the _H2 separation column.

In addition, however, it should also be possible, as an alternative or in addition to the gaseous material stream, to withdraw a low-methane material stream in liquid form from the _H2 separation column and feed it to the CO/ _CH4 separation column as reflux. In this case, cooling of the low-methane material stream before its introduction into the CO/ _CH4 separation column is preferably omitted. This process variant can be used with particular advantage when an _H2 separation column is used which is designed as a dividing wall column in its lower section. This makes it possible to obtain a low-hydrogen material stream in liquid form which has a significantly lower methane content than a gaseous material stream with the same hydrogen content, so that a significantly higher purity of the carbon monoxide-rich gas phase can be achieved in the CO/ _CH4 separation column.

The CO/ _CH4 separation column is preferably operated at a pressure that allows the carbon monoxide-rich gas phase, after warming against process streams to be cooled, to be delivered to a customer at a pressure equal to or greater than that required by the customer for the carbon monoxide product. The CO/ _CH4 separation column is preferably operated at a pressure between 8 and 10 bar(a).

Developing the method according to the invention, it is proposed to provide cooling, in particular the peak cooling required for the process, via a cooling circuit in which nitrogen is used as the refrigerant. The nitrogen circuit has no connection to a flammable and/or toxic process gas, so that a compressor that is neither explosion-proof nor operated in a specially protected zone is expediently used to drive it.

Furthermore, the invention relates to a device for the cryogenic decomposition of a feed gas containing methane and consisting predominantly of hydrogen and carbon monoxide, comprising at least one heat exchanger for cooling and partial condensation of the feed gas, a separator in which a first liquid phase can be separated from the partially condensed feed gas, an H ₂ separation column which can be heated by a circulation heater and in which the first liquid phase by separating hydrogen, a second liquid phase can be produced, as well as a CO/CH ₄ separation column in which a carbon monoxide-rich gas phase can be separated from the second liquid phase with a purity that allows its release as a carbon monoxide product.

The stated object is achieved according to the invention in that the H ₂ separation column is connected to the CO/CH ₄ separation column in such a way that a low-methane stream can be withdrawn from the H ₂ separation column via a withdrawal point and fed to the top of the CO/CH ₄ separation column as reflux.

In order to liquefy a gaseous stream withdrawn from the _H2 separation column before it is introduced into the CO/ _CH4 separation column, the invention provides a cooling device arranged between the two separation columns. The cooling device is preferably a heat exchanger that is also used for cooling and/or partially condensing the feed gas. However, the cooling device may also be designed as a standalone heat exchanger.

The _H2 separation column comprises several mass transfer devices arranged vertically one above the other, representing practical separation stages. These are preferably designed as sieve trays and/or slotted bubble trays and/or structured packings and/or random packings. Below the practical separation stages is the column's bottom zone, to which heat can be supplied via the circulation heater.

If the low-methane stream is to be withdrawn from the _H2 separation column in gaseous form, the withdrawal point is preferably located below the sixth practical separation stage of the first separation column. It is particularly preferably located between the bottom zone and the third practical separation stage.

In a suitable embodiment of the invention, the H ₂ separation column has a vertical dividing wall in its lower section, which divides the column cross-section into two segments. At the upper end of the dividing wall, there is an inlet point through which a portion of the first liquid phase can be introduced into one of the segments, as well as a cooling device for condensing Gas that rises from the bottom chamber via the other segment. This device, referred to as a dividing wall column, enables the production of a low-methane liquid phase, which, due to its composition, can be used as reflux in the CO/ _CH4 separation column. For this purpose, the dividing wall column is designed with a withdrawal point, preferably located directly below the cooling device, through which a low-methane stream can be withdrawn in liquid form and fed to the CO/ _CH4 separation column via a liquid line. In the simplest case, the liquid line is designed as a pipeline and, logically, does not include any device for cooling the low-methane liquid phase.

A particularly preferred variant of the device according to the invention provides a cooling circuit that can be operated with nitrogen as a refrigerant and runs through the heat exchanger(s) for cooling and partially condensing the feed gas, via which cooling circuit, in particular, the peak cold required at the separator for gas separation can be provided. To drive the nitrogen that can be carried in the circuit as a refrigerant, the cooling circuit expediently comprises a non-explosion-proof compressor with fewer than three compressor stages. Furthermore, the cooling circuit can have a supply device arranged on the suction side of the compressor for introducing gaseous nitrogen into the circuit, as well as a removal device for removing excess nitrogen from the circuit, which is located on the pressure side of the compressor. The cooling circuit preferably comprises a further heat exchanger for condensing gaseous nitrogen, into which the circulation heater of the CO/CH4 separation column is integrated.

In the following, the invention will be described with reference to two examples Figures 1 and 2 schematically illustrated embodiments will be explained in more detail.

The Figure 1 shows an embodiment of the process according to the invention in which a material stream intended as reflux for the CO/CH ₄ separation column is withdrawn in gaseous form from the H ₂ separation column.

The Figure 2 shows another embodiment of the process according to the invention, in which a material stream intended as reflux for the second CO/CH ₄ separation column is withdrawn in liquid form from the H ₂ separation column.

In both figures, identical plant components and process streams are identified by the same reference symbols.

In Figure 1 A feed gas 1 to be separated, consisting predominantly of hydrogen and carbon monoxide and containing methane, which is present at a pressure of between 30 and 60 bar(a), is cooled in the first heat exchanger E1 and in the second heat exchanger E2 against process streams that are to be heated, whereby the condensation of components creates a two-phase mixture 2, which is separated in the separator D1 into a liquid phase consisting largely of carbon monoxide and methane and containing hydrogen and a hydrogen-rich gas phase. The gas phase is withdrawn from the separator D1 via line 3 and, after heating in the heat exchangers E2 and E1, is released as raw hydrogen 4 at the plant boundary. The liquid phase 5, on the other hand, is fed to the H ₂ separation column T1. For this purpose, it is split into two partial streams, of which the first 6 is expanded as reflux to the top of the H ₂ separation column T1, while the second partial stream 7, after expansion and partial evaporation in the heat exchanger E2, is fed to the middle section of the H ₂ separation column T1 as intermediate heating.

The _H2 separation column T1 is operated at a pressure between one-third and one-half of the pressure of the feed gas 1 and serves to remove the hydrogen dissolved in the liquid phase 5. It is heated by a circulation heater 8 integrated into the heat exchanger E2.

The hydrogen-rich overhead fraction 9 from the _H2 separation column T1 is heated in the heat exchangers E2 and E1 and released as flash gas 10 at the plant boundary, while the largely hydrogen-free bottom fraction 11, consisting of carbon monoxide and methane, is expanded into the CO/ _CH4 separation column T2, which is operated at a pressure between 8.5 and 9 bar(a). For this purpose, the bottom fraction 11 is split into two partial streams, one of which, 12, serves as intermediate reflux, and the second, 13, after evaporation in the heat exchanger E2, serves as intermediate heating. The CO/ _CH4 separation column T2 is heated by a circulation heater 14 integrated in the heat exchanger E3.

The peak cooling required for the process is obtained via a nitrogen circuit driven by the two-stage cycle compressor V. Nitrogen 15 leaves the second compressor stage C2 at a pressure typically between 16 and 21 bar(a), is subsequently cooled in heat exchanger E1 and condensed in heat exchanger E3 against bottom product 14 from the CO/ _CH4 separation column T2, which is to be heated. The condensed nitrogen 16 is expanded to an intermediate pressure between 7 and 9 bar(a), forming a two-phase mixture 17, which is separated in separator D2 into a gas phase 18 and a liquid phase 19. A material stream 21 formed from the gas phase 18 and a portion 20 of the liquid phase 19 is completely evaporated at the intermediate pressure level in heat exchanger E2 and further heated in heat exchanger E1 before being fed to the suction side of the second compressor stage C2. The remaining liquid phase 22 is further expanded to a low pressure level between 3 and 5 bar(a), evaporated in the heat exchanger E2, and, after being heated in the heat exchanger E1, returned to the cycle compressor V via the suction side of the first compressor stage C1. The liquid phase 19 is divided into the two partial streams 20 and 22 in such a way that the temperature required at the separator D1 is reached.

If necessary, nitrogen can be added to the closed nitrogen circuit from the outside via the low-pressure passage 22, with gaseous nitrogen 23 being introduced on the warm side of heat exchanger E1 and liquid nitrogen 24 on the cold side of heat exchanger E2. Excess nitrogen 25 is discharged on the pressure side of the circuit compressor V.

To generate a reflux for the CO/CH ₄ separation column T2, a methane-poor gas phase 26 is withdrawn from the H ₂ separation column T1 below the sixth practical separation stage, cooled and condensed in the heat exchanger E2, and then fed via line 27 to the top of the CO/CH ₄ separation column T2. The top product 28 of the CO/CH ₄ separation column T2 has the purity required for a carbon monoxide product and is present at a pressure high enough to be released as a carbon monoxide product 29 after heating in the heat exchangers E2 and E1 without further compression. In the bottom of the CO/CH ₄ separation column T2, a methane-rich, carbon monoxide-containing liquid phase 30 collects, which after evaporation and heating in the heat exchangers E2 and E1 is released as fuel gas 31.

The Figure 2 The embodiment shown allows the carbon monoxide product 29 to be produced with a higher purity than is possible with the Figure 1 shown configuration is possible. For this purpose, a column T3 is used to strip hydrogen from the liquid phase 5, which is divided in its lower region by a dividing wall into two segments S1 and S2. At the upper end of segment S1 is the feed point for the partial stream 7 of the liquid phase 5, which serves as an intermediate heater, while at the upper end of segment S2 there is a condenser E4, in which a portion 32 of the bottom fraction 11 consisting of carbon monoxide and methane is used as coolant. The warmed and evaporated coolant 33 is subsequently fed together with the partial stream 13 to the CO/ _CH4 separation column T2 as an intermediate heater. In order to avoid methane contamination of the liquid phase in segment S2, the liquid phase flowing out of the upper region of column T3 is fed alone to segment S1. Below the condenser E4, a low-methane carbon monoxide fraction 34 can therefore be withdrawn in liquid form from the segment S2, which serves as reflux at the top of the CO/CH ₄ separation column T2.

Claims (14)

1. Method for the cryogenic decomposition of a feed gas (1) containing methane and consisting predominantly of hydrogen and carbon monoxide, which feed gas is partially condensed by cooling in order to obtain a first liquid phase (5) containing hydrogen and consisting largely of carbon monoxide and methane, from which first liquid phase a second liquid phase (11) is produced by the separation of hydrogen (9) in a H₂ separation column (T1) heated by a circulation heater (8), from which second liquid phase a carbon monoxide-rich gas phase (28) is obtained in a CO/CH₄ separation column (T2) having a purity that allows it to be released as a carbon monoxide product (29), characterized in that a low-methane stream (26, 34) is withdrawn from the H₂ separation column (T1) and then is fed as reflux at the top of the CO/CH₄ separation column (T2).
2. Method according to claim 1, characterized in that the low-methane stream (26) is withdrawn in gaseous form from the H₂ separation column (T1) and is liquefied by cooling before its introduction into OO/CH₄ separation column (T2).
3. Method according to either claim 1 or claim 2, characterized in that the low-methane stream (26) is withdrawn from the H₂ separation column (T1) below the sixth practical separation stage.
4. Method according to claim 3, characterized in that the low-methane stream (26) is withdrawn from the bottom space and/or between the first and the third practical separation stage.
5. Method according to claim 1, characterized in that the low-methane stream (34) is withdrawn in liquid form from the H₂ separation column.
6. Method according to any of claims 1 to 5, characterized in that the carbon monoxide-rich gas phase (28) contained in the CO/CH₄ separation column is heated and released as a carbon monoxide product (29) without an increase in the pressure.
7. Method according to any of claims 1 to 6, characterized in that peak cooling is provided by a nitrogen cycle.
8. Device for the cryogenic decomposition of a feed gas (1) containing methane and consisting predominantly of hydrogen and carbon monoxide, having at least one heat exchanger (E1, E2) for cooling and partially condensing the feed gas (1), a separator (D1) in which a first liquid phase (5) can be separated from the partially condensed feed gas (2), a H₂ separation column (T1) which can be heated by a circulation heater (8) and in which a second liquid phase (11) can be produced from the first liquid phase (5) by the separation of hydrogen (9), and a CO/CH₄ separation column (T2) in which a carbon monoxide-rich gas phase (28) can be separated from the second liquid phase (11) that has a purity that allows its release as a carbon monoxide product (29), characterized in that the H₂ separation column (T1) is connected to the CO/CH₄ separation column (T2) in such a way that a low-methane stream (26, 34) can be withdrawn from the H₂ separation column (T1) via an extraction point and fed as reflux at the top of the CO/CH₄ separation column (T2).
9. Device according to claim 8, characterized in that a cooling means (E2) is arranged between the H₂ separation column (T1) and the CO/CH₄ separation column (T2) for liquefying a low-methane stream (26) withdrawn in gaseous form from the H₂ separation column.
10. Device according to either claim 8 or claim 9, characterized in that the extraction point for the low-methane stream (26) is arranged below the sixth practical separation stage of the H₂ separation column.
11. Device according to claim 10, characterized in that the extraction point for the low-methane stream (26) is arranged between the bottom space and the third practical separation stage of the H₂ separation column (T1).
12. Device according to claim 8, characterized in that the H₂ separation column (T1) is designed in its lower part as a dividing wall column from which the low-methane stream (34) can be withdrawn in liquid form.
13. Device according to any of claims 8 to 12, characterized in that it comprises a cooling circuit that can be operated with nitrogen as a coolant.
14. Device according to any of claims 8 to 13, characterized in that the H₂ separation column (T1) is has a sieve tray and/or slotted bubble cap tray and/or structured packing and/or packed beds as practical separation stages.